Description of the Prior Art
This invention, in its preferred form, relates to apparatus for use in the welding of a fuel rod grid of Zircaloy straps by laser beam welding, and more particularly, the invention relates to welding plates which are positioned on the grid to hold the straps forming the grid in proper alignment and orientation, as well as holding vanes forming parts of the straps.
Nuclear fuel bundle assemblies include a matrix of nuclear fuel rods which are arrayed in rows and columns, and which are held in the desired configuration by a plurality of fuel rod grids. These grids are produced from "straps" which are linearly extending, generally rectangular elements, characterized by having slots extending from one edge approximately half way through the depth of the strap. The straps are assembled so that one strap is in mating relationship with the other strap. Thus, the slot of one strap engages the other strap at a portion thereof which is in alignment with the slot of that other strap, with the result that the grid is of the same depth as each of the straps which forms the grid. The resulting grid has a first set of straps which are substantially parallel to each other, and equally spaced, and a second set of straps which are parallel to each other and equally spaced, the straps of one set being perpendicular to the straps of the other set. All of the aforesaid straps are designated as "inner straps", and they are placed in mating relationship to form a square grid of square cells, in the above noted rows and columns. In addition, there are provided outer straps, which are placed on the four sides of the grid.
The inner straps and outer straps were provided with vanes, which extended from them at a substantial angle, for the purpose of causing water passing through the cells to be given a swirling, turbulent motion. Once the inner and outer straps were assembled to form a grid, brazing material was supplied at juncture points, and the assembled grid, with the brazing material, was positioned in an oven, where brazing was effected and thereby the joining together of the straps to form a sturdy grid.
The above-described method was satisfactory, but it involved the utilization of straps of Inconel. It has been discovered that Inconel results in less efficient use of the nuclear fuel than is desirable, and so there has been developed fuel rod grid straps made of Zircaloy. This material has the advantage that it permits a more efficient use of the nuclear fuel. However, Zircaloy cannot be joined by brazing, since it cannot be exposed to the temperatures necessary for brazing. As a result, it was found necessary to go to a technique of welding the Zircaloy straps in which a laser beam was utilized. The utilization of a laser beam for welding a grid has produced new requirements, including the necessity for precise positioning of each point of intersection of the grid, formed by a pair of intersecting grid strips, as well as insuring the parallel and equal spacing of the grid straps of each set of grid straps, the perpendicularity of the grid straps of each set relative to the grid straps of the other set, and then insuring that the edges of the grid straps are straight. In addition, Zircaloy is known to be a material which cannot be subjected to repeated bending stresses, and therefore, since the grid straps on one side are provided with vanes, it is necessary to hold the vanes, to prevent their being "turned under" and thereby risking their being broken off. Further, the use of welding beam techniques requires not only the holding of the grid straps as hereinabove described, but also requires that the laser beam have access to each of the locations to be welded, particularly to the noted points of intersection of the welding straps.